Neural tissues of various types obtained from the rat embryos of different developmental ages will be transplanted homo- as well as heterotopically in the brains of the neonatal rats. The survival, growth and differentiation of the transplants will be studied with the aid of thymidine-H3 autoradiography, Nissl stain, Golgi-Cox impregnation, and electron microscopy. Some of the major problems to be investigated will be: developmental analyses of neural transplants from different regions of the embryos, relationship between the age of the donor and growth of the transplants, the age of the host animals and growth of the transplants, cellular and cytoarchitectural differentiation in the transplants, and synaptogenesis in the transplants. In addition to the above investigations, major attention will be focused on the problems of anatomical integration of transplants with the host brain. This will involve study of afferents and efferents of the transplants. Problems related to degeneration in the host neural structures due to the growth of the neural transplants will also be studied. Since these studies are developmental-anatomical in nature, the normal material obtained from embryos and neonatal animals will be used as controls.